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# Change in internal energy

Physics and electronics discussion, revision, exam and homework help.

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1. Change in internal energy
Going over past papers and having problems with this question.

"A boiler heats high pressure water from 80C to 152C and then evaporates it without raising the temperature. The specific heat capacity is 4.86 kJ/kg K and the latent heat Ufg is 1923 kJ/kg. Calculate the change in internal energy of 1 kg."

Would be greatful if somone could give me some advice or show me how they would answer it.

thanks
Last edited by t.ellingham; 23-04-2012 at 13:26.
2. Re: Change in internal energy
You need to use m c delta t + ufg
3. Re: Change in internal energy
(Original post by mmneedtokill)
You need to use m c delta t + ufg
1kg x 4.86 x 72 + 1923 = 2271.92

Is this anywhere close?

4. Re: Change in internal energy
(Original post by t.ellingham)
1kg x 4.86 x 72 + 1923 = 2271.92

Is this anywhere close?

Typo. It should be 720
5. Re: Change in internal energy
(Original post by Ari Ben Canaan)
Typo. It should be 720
sorry, tsr converted the degrees sign into a 0, the values are actually, 80C and 152C

is my answer at all right?
6. Re: Change in internal energy
(Original post by Ari Ben Canaan)
Typo. It should be 720
is my answer right if i use 72?
7. Re: Change in internal energy
(Original post by t.ellingham)
is my answer right if i use 72?
I suspect so unless I'm missing something..
8. Re: Change in internal energy
(Original post by t.ellingham)
1kg x 4.86 x 72 + 1923 = 2271.92

Is this anywhere close?

Your current answer is in kj/s but I'd convert it to MW
9. Re: Change in internal energy
(Original post by mmneedtokill)
Your current answer is in kj/s but I'd convert it to MW
if i convert it to MW, giving me 2.27 MW. would this be the correct answer?

10. Re: Change in internal energy
(Original post by t.ellingham)
if i convert it to MW, giving me 2.27 MW. would this be the correct answer?

The other user is wrong. You cannot convert your answer to watts as internal energy is measured in JOULES not J/s.

That is, internal energy is NOT a measure of power.

Keep it in joules or KJ....

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Last updated: April 25, 2012
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